Trying to create a scripting agent to change calendar permissions on-create.

I've been asked to change the way I update calendar permissions for new mailboxes so that permissions are set "on-create" rather than via a nightly script.

So I enabled the scripting agent and I came up with

<?xml version="1.0" encoding="utf-8" ?><Configuration version="1.0"><Feature Name="MailboxProvisioning" Cmdlets="new-mailbox"><ApiCall Name="OnComplete">
   if($succeeded)    {
    $newmailbox = $provisioningHandler.UserSpecifiedParameters["samaccountname"]
    Set-MailboxFolderPermission -User "Default" -AccessRights Reviewer -Identity $newmailbox":\Calendar"
   }</ApiCall></Feature></Configuration>

However it errors out each time it runs.   What am I missing here?